Cistern



FIG. 1 is a Top View of a cistern showing my new design;

FIG. 2 is a Left hand side View thereof;

FIG. 3 is a Front View thereof;

FIG. 4 is a Right hand side View thereof;

FIG. 5 is a Rear View thereof;

FIG. 6 is a Bottom View thereof; and,

FIG. 7 is a Perspective View thereof. 

The ornamental design for a cistern, as shown and described. 